Ischemic heart disease and cardiomyopathy are major cardiac disorders leading to sudden death in adults in the United States. Death usually results from ventricular tachycardia degenerating into fibrillation. Surgical and trans-catheter ablation therapies have emerged from the treatment of such arrhythmias. These treatments require activation mapping of the ventricles to locate aberrant electrical pathways prior to surgical excision or ablation. Intraoperative mapping is currently the method of choice for locating such pathways. Intraoperative mapping procedures, however, are complex, time consuming, and invasive. There is, as a result, a considerable incentive to minimize their extent and necessity. A non-invasive mapping technique capable of locating aberrant electrical pathways prior to surgery, would be of considerable value. The purpose of this proposal is to develop such a technique based upon the detection of backscattered x- rays. Backscattered x-rays will be used to create high precision displacement, velocity and acceleration maps of the free walls of the heart. These high temporal resolution maps will then be used to locate site of electrical ectopic activity. The backscatter mapping procedure is non-invasive, requires neither cutdowns nor injections, and uses minimal levels of radiation. The specific aims of the proposal are: a) to develop the necessary hardware and software for measuring high precision wall dynamics using a radiation backscatter measurement technique, and b) to use that hardware and software for activation mapping studies in a paced canine model of ventricular arrhythmias. The end goal of this proposal is a non-invasive high precision activation mapping technique suitable for initiating testing in humans.